1. Field of the Invention
The invention relates to a semiconductor photodetector structure, especially to a photodetector structure that includes germanium semiconductor material.
2. Description of the Prior Art
Conventional copper cables are less and less capable of carrying more and more signals to travel a longer and longer distance due to the physical limitation of electrical resistance and signal delays. Naturally, optical fibers meet the demand of carrying very large information to travel a very long distance so they replace the conventional copper cables to be the medium of long distance carrier of information because one single optical fiber allows multiple beams of light of different wavelengths, each carrying different information to travel at the speed of light without mutual interference and without attenuating too much after traveling an extreme long distance.
Light of different wavelengths in the form of pulse signals constitutes the basic principle of transmission by optical fiber. However, such basic principle of transmission is not compatible with the basic principle of transmission by electron current in the current electronic devices to carry and to transmit signals. In order to from a transform medium between the optical fiber transmission and the electron current transmission, the photo-detector is deemed to be a convenient tool.
The photo-detector is an important photo-electrical transform unit. The photo-detector is capable of transforming the optical signals to electrical signals (into voltage or current), so it can transform the optical pulse signals in the optical fibers to become the electrical signals which can be carried, transmitted or used by ordinary electronic devices. Amongst them, the PIN (p-intrinsic-n photodiode) which has the advantages of easy to be manufactured, high reliability, low noise, compatible with low-voltage amplifier circuits and very wide bandwidth becomes one of the most widely used photo-detector.
The basic operational mechanism of the PIN photodiode is that when the incident light hits the p-n junction of the semiconductor, the electrons in the valence band of the semiconductor would absorb the energy of the photons in the incident light and jump over the forbidden band to arrive at the conduction band, which means, the incident photons create electrons, called photo-electrons, in the conduction band of the semiconductor if the photons have sufficient energy. Simultaneously, an electrical hole is left behind in the valence band and an electron-hole pair, or called photocarrier, is thus generated, which is also known as the photoelectric effect of the semiconductors. Afterwards, the photo-electron and the corresponding hole are quickly separated under the influence of an inner electric field and an outer negative bias to be respectively collected at the positive electrode and the negative electrode. Therefore, a photo-current appears in the outer circuit.
In order to enhance the operational performance of the PIN photodiode, the current technology integrates the Ge semiconductor material into the Si substrate to accomplish an optical communication of wide wavelength because Ge is deemed to have much higher carrier mobility than Si. The importance of integration of Ge semiconductor material into the Si substrate lies in the essential qualities of fast, effective and low noise. US Patent Publication 2004/0043584 discloses a method to integrate the CMOS and Ge photodetector on a silicon substrate. The method is to form a plurality of shallow trenches, and after deposition of a germanium layer, an etching and a CMP step are performed to form a germanium photodetector on the silicon substrate. However, the germanium layer is non-selectively deposited on the substrate, and should be etched and planarized, making the roughness of the germanium layer hard to control and lowering the photodetecting quality. U.S. Pat. No. 7,220,632 and U.S. Pat. No. 7,262,117 disclose a germanium photodetector fabricating method as well. However, the direction of the light input and the diode channel is horizontal, which may cause more noise. And the photodetector is fabricated on a SOI substrate, which increases the manufacturing cost.
Accordingly, it is needed to provide a novel germanium photodetector structure and the method for making the germanium photodetector to more effectively integrate the manufacturing process of the novel germanium photodetector with the traditionally fully-developed MOS manufacturing process to lower the manufacturing cost.